1. Field of the Invention
This invention relates to the reproduction of an optical image, and particularly to a reflection mask for X ray illuminated by reflected illumination having the wavelength (about 2-100 nm) of a soft X ray range.
2. Related Background Art
A mask used for photoetching or the like has heretofore been formed with a predetermined pattern comprising reflecting portions and non-reflecting portions for illuminating light. This pattern has been projected onto the surface of photosensitive film by a mirror optical system (see, for example, J. Vac. Sci. Technol. B7 (1989), p. 1648).
It is known that when a pattern provided on such a projected type reflection mask (hereinafter referred to as the reflection mask) is projected by a mirror, there is a limit of resolution in the projected reproduced image due to a diffraction phenomenon. In this case, a cut-off frequency as an ideal limit value is 2 NA/.lambda. (lines/mm) for incoherent illumination and NA/.lambda. (lines/mm) for coherent illumination when the numerical aperture of the projection mirror is NA and the wavelength of illuminating light used is .lambda.. Accordingly, to enhance the resolution for illuminating light having a certain wavelength, it has been necessary to make the numerical aperture NA of the projection mirror great.
Generally, however, a mirror optical system is greater in aberration than a lens optical system and if the numerical aperture thereof is made greater than a certain value, the expanse of an image by aberration will exceed the resolution limit by diffraction. Therefore, even if an attempt is made to make the numerical aperture of the mirror system in order to a desired resolution, the magnitude of the numerical aperture has been limited.
FIG. 6 of the accompanying drawings shows a schematic cross-section of a prior-art reflection mask (for visible light) and the state of the amplitude distribution of the light of a projected image by this reflection mask. As shown, in the projected image of a prior-art ordinary reflection mask 9, comprising reflecting portions 1 and non-reflecting portions 2 alternately arranged on a base plate 8, the light from the reflecting portions 1 spreads to the regions corresponding to the non-reflecting portions 2 due to a diffraction phenomenon. Therefore, there is created a limit in the spacing .DELTA. between the reflecting portions 1 which can be discriminated. Such a phenomenon is to be found not only in visible light but also in X ray. Therefore, a similar problem has also arisen in a reflection mask for X ray.